Always Primary PAN first. Then the next PAN, PSNs last. I can't remember seeing a documented technical reason why this order is good/necessary. I think it's possibly just common sense because the primary database is on the Primary Admin node and since it's in charge of writing to the database, it gets patched first. The operational benefit of doing PAN first, is that you "take the hit" of taking down the admin GUI first, and then you can monitor the rest of the patching process from the GUI.
